The invention relates to a press-fit connection between prosthesis components of joint prostheses in accordance with the preamble of the first claim.
Joint prostheses, in which one joint partner is formed as a socket and the other joint partner is formed as a spherical head which is rotatably mounted in the socket, are known in particular as shoulder-joint and hip-joint prostheses. These prostheses, as a rule, are built up in a modular fashion. Hip-joint endoprostheses, for example, consist of the socket, which is inserted into the hip bone, and of the shaft, which is inserted into the femur. The socket consists, as a rule, of a metallic outer shell into which a shell insert made of ceramic material or a biocompatible plastics material is inserted. A press-fit connection is used here as the connecting technique. A press-fit connection is known, for example, from DE 196 11 248A1. The shaft has a peg, the so-called cone, onto which the spherical head is slipped. In the case of the modularly constructed endoprostheses, implant components are connected together that are made of different materials and are of different sizes for the purposes of adaptation to the physique of the patient. For example, spherical heads made of a cobalt-chromium alloy or made of an aluminium-oxide ceramic material are slipped onto a cone made of titanium. The press-fit connection, in particular conical pressing, is also used here as the connecting technique between metallic or ceramic spherical heads and the cone. In this connection, the spherical head which has a conical bore is placed upon the cone. After the spherical head has been slipped onto the cone, fixing is effected by hitting the spherical head.
As is known from the publication xe2x80x9cDas Prinzip der Konus-Steckverbindung fxc3xcr keramische Kugelkxc3x6pfe bei Hxc3xcftendoprothesenxe2x80x9d by G. Willmann, Mat-wiss. u. Werkstofftech. 24, 315-319 (1993), all ceramic materials are brittle, that is to say, they are sensitive to non-areally introduced loads, that is, stress concentrations. The contact surfaces between the bore in the ceramic ball and the surface of the cone must be matched to each other in an optimum manner. The percentage bearing area proportion of the surfaces that is provided must be the maximum possible. It has been possible-to achieve this by making extremely high demands with respect to the dimensional tolerances of the bore and of the metal cone. However, since that cannot be realized technically and is not expedient economically, the potential of the plastic deformability of the metals is exploited by structuring the surface of the metallic cone. For example, the surface is roughened to a great extent or is provided with a specific structure, for example it is provided with a groove. If the spherical head is now slipped onto the metallic cone, the structured surface is deformed and the percentage bearing area proportion of the surfaces is increased. As a result, the stress concentration is minimized and in addition the friction is increased so the protection against torsion is also improved. The greater the roughness-height that lies in the micrometer range is, the higher the tolerable breaking load is. Since, however, the ceramic material, is harder than the metallic material of the cone, in particular in the case of load-related relative movements between the cone and the spherical head, it is possible that abrasion will occur on the surface of the cone and loosening of the connection will take place. Manifestations of wear as a result of relative movements are also possible in the case of joint sockets where the shell insert is held in the outer shell by means of a press-fit.
The object of the present invention is to propose an improved press-fit connection between the prosthesis components of joint prosthesis, i.e., between a metallic component which is anchored in the bone and a ceramic or biocompatible plastic component forming a friction partner of the joint.
The object is achieved with, by providing the ceramic or biocompatible plastic component with a covering made from a biocompatible metal or biocompatible metal alloy on its pressing surface by way of which it is connected to the metallic component.
The covering in accordance with the invention made from a biocompatible metal or a biocompatible metal alloy on one of the pressing surfaces of the components, the inner surface of the conical bore of the spherical head or the outer cone section of the shell insert respectively, acts as an adhesion promoter between the spherical head and the cone of the shaft or the shell insert and the outer shell of the joint socket respectively. The covering can be produced to any thickness that is desired. Electrolytic deposition or deposition out of the gas phase (sputtering) is possible. A preferred method of application is spraying, with plasma-spraying being particularly suitable for spraying metals or metal alloys that have high melting points. Plasma-spraying presents the possibility of being able to produce a coating to the required thickness by adjusting the distance of the flame and the temperature of the flame. What is important in the case of all the coating methods and coating materials is that there is good adhesion between the coating material and the surface of the prosthesis components made from ceramic or plastics material. This adhesion can be effected by means of a form of mechanical clamping in the rough surface of the ceramic material or plastics material or by connecting the material which is applied to the corresponding material in the boundary layer. This is possible in particular when plasma-spraying metallic materials that have high melting points, for example in the case of titanium alloys.
When pressing the spherical head onto the cone and also when pressing the shell insert into the outer shell, in addition to the frictional adhesion as a result of the great pressure on the covering, it is possible for a connection to be established between the material of the covering and the metallic material of the pressing surface of the cone or the outer shell respectively, for example in the form of a so-called cold-weld, by means of which the adhesion of the two components can be increased.
The adhesion of the prosthesis components made from ceramic material or plastics material to their metal partners, the adhesion of a spherical head to a cone and also of a shell insert to an outer shell, is advantageously increased owing to the fact that the covering has a rough surface. When a spherical head is pressed onto the cone of a shaft as well as when a shell insert is pressed into an outer shell, the tips of the rough surface of the covering are levelled, whereby a large percentage bearing area is produced for the frictional adhesion. The surface of the covering may already be rough on account of the method of application itself, for example plasma spraying, yet can also be brought to the desired height of roughness by means of subsequent treatment, for example sand blasting.
A corresponding increase in material develops as a result of the covering in the region of the pressing surfaces, on the lateral surface of the conical bore or the outer cone section respectively. It is therefore advantageous if the dimensions of the prosthesis components in the region of the pressing surfaces are selected with regard to the dimensions with the covering that are required for a press-fit so that the covering can be applied to an optimum thickness and with an optimum roughness-height matched to the respective pairing of components. Moreover, advantageously as a result of the covering on one of the pressing surfaces, the possibility exists of compensating for tolerances or deviations from the straightness and roundness of the cone and conical bore in the spherical head or shell insert and inner cone section in the outer shell, whereby the risk of relative movements under loading as a result of a bad fit is precluded.
Depending on the material, method of application and layer thickness, the covering can have a roughnesses of 20 xcexcm to 90 xcexcm depth. The depths of roughness of the pressing surfaces of the components that are to be connected together by means of adhesive friction have a decisive influence upon, in particular, the breaking load of the ceramic component. With an increasing depth of roughness of the pressing surface of the metallic partner in conventional connections, the breaking load of the ceramic component increases. In order to guarantee a solid and durable fit of the components in accordance with the invention, advantageously, the toughness depth of the covering should, if possible, not drop below substantially 20 xcexcm. With a decreasing roughness depth, the resistance against levelling of the peaks increases and a comparatively high level of force must be used.
A roughness depth of between 60 xcexcm and 90 xcexcm has proved to be particularly advantageous. This is due to the fact that consequently peaks of a sufficient height are provided that are levelled when a spherical head is mechanically pressed onto a cone or when a shell insert is pressed into an outer shell and which, with increasing levelling, result in an increasing percentage bearing area. With an increasing percentage bearing area, the friction and therefore also the security against relative movements, in particular torsion, of the spherical head and cone, or the shell insert and outer shell, increase.
Spherical heads and shell inserts made from ceramic material have proved to be advantageous not just because of their wear resistance. Since ceramic materials do not have any corrosive effect upon metals or metal alloys, they can quite safely be provided with a covering of a biocompatible metal or a biocompatible metal alloy.
Furthermore, it is advantageous if the material of the covering and the material of the pressing surface of the cone or the outer shell correspond. It is known that in the case of implant components made from different materials, for example where a spherical head of cobalt chromium is slipped onto a shaft made from titanium alloys, on account of relative movement between the portions and the presence of the body fluid acting as an electrolyte, so-called fretting corrosion is possible. On account of the micro-movements that occur between the spherical head and cone as a result of the body load, it is possible for detectable abrasion to occur that results in damage to the protective oxide layer of the otherwise biocompatible metals or metal alloys. The so-called fretting corrosion is caused by the resultant galvanic reactions of the different metal components rubbing against each other.